CHEMISTRY OF FIRST CYCLE SEDIMENTS AS CLIMATIC AND STRATIGRAPHIC TOOLS

First – cycle Gondwana sediments in the intra-cratonic Raniganj Basin, India, range from the Late Paleozoic to Early Mesozoic. The sediments, derived from Precambrian granitic source were deposited within fluvio-glacial and fluvial systems during climate change from an Icehouse to Greenhouse state. During sedimentation, the source rock and environment of deposition remained uniform. Climate, through chemical weathering, controlled the nature of mineralogy and chemistry of sediment and generated four sedimentary facies: tillite-sandstone-shale (Facies A); sandstone-shale-coal (Facies B); red/green shale-sandstone (Facies C); sandstone-conglomerate (Facies D). Chemical parameters of first cycle sediments are useful as paleoclimate indicators and stratigraphic tools because climate and sediment chemistry are directly related.

Twenty-three sandstone and 26 shale samples were analyzed to test this hypothesis.

A chemical maturity pattern in sandstone expressed by the parameter SiO2/Al2O3, discriminates different facies shown with their average values as follows: Facies A(6.2), Facies B(7.8), Facies C(7.2), Facies D(28.5). In the case of shale, the best discrimination is observed based on a maturity parameter defined as Al2O3/(K2O + Na2O + CaO + MgO). Average values of this parameter also show a similar pattern as above: Facies A(2.0), Facies B(3.6), Facies C(1.6), Facies D (15.7). In both cases Facies A and Facies C seem to be immature while Facies B is mature and Facies D is most mature.

The observed trends are interpreted as follows: glacial (Facies A), temperate humid (Facies B), warm semi-arid (Facies C), warm humid (Facies D). Since the chemical parameters change at the facies boundaries, they can also be used as a stratigraphic tool.

In addition, trace elements, in both sandstone and shale, were analyzed. Based on trace elements no discrimination between facies is seen except in Facies D where Ba, Co, Cr, Ga, Rb, Sr, Mn, Ni, and Pd show low concentration. Higher concentration and wide scattering of these elements characterize Facies A, B, and C. Low concentration of trace elements in facies D may be related to extreme chemical weathering under warm humid condition.

This study shows that except under extreme warm humid climate trace elements may not be useful as climatic and stratigraphic tools.